GRX (GPRS Roaming Exchange)
Updated
The GRX is the private IP network, separate from the internet, that operators first used to carry roaming data between them for 2G and 3G. IPX is its successor, with service level guarantees and more services. Both are described in GSMA IR.34.
In practice
In a home-routed roaming session, device data crosses the GRX or IPX from the visited network to the home network before reaching any private delivery link.